Characteristics of [3H]inositol(1,3,4,5)tetrakisphosphate recognition sites in human cerebellar membranes

The characteristics of specific [3H]Ins(1,3,4,5)P4 binding sites in human cerebellar membranes were determined in this study. Binding rapidly reached steady state, possessed a pH optimum of 4.5-5.1 and was greater in the absence of BSA than in its presence. Heparin inhibited both specific and pseudospecific binding of the ligand, whereas only the specific binding was inhibited by non-radioactive Ins(1,3,4,5)P4. Calcium at a concentration of 1 mM, reduced binding by 27%. Competition studies with other inositol phosphates showed specificity for Ins(1,3,4,5)P4 with a pI50 value of 6.87 and a Hill coefficient of 0.27, indicating two sites. Ins(1,2,5,6)P4, Ins(1,3,4,6)P5, Ins(3,4,5,6)P4 displaced binding with IC50 values ranging from 0.1-1 microM, Ins(1,2,5,6)P4 and Ins(1,3,4,5,6)P5 being the most potent. Ins(1,4)P2 and Ins(1,5,6)P3 had lesser effects on binding. Rosenthal analysis of [3H]Ins(1,3,4,5)P4 saturation binding data at low ligand concentrations gave a KD of 27 nM and a Bmax of 33 pmol/mg protein. It is concluded that [3H]Ins(1,3,4,5)P4 binding sites in human cerebellar membranes have similar characteristics to these sites reported in the literature in animal cerebellar tissue, but are in greater abundance.